1. Field of the Invention
This invention pertains to the improvement of the resolution of processed electronic data delivered from the heterodyne detectors of ring laser gyroscopes or angular rate sensors, and more particularly to enhanced resolution multioscillator ring laser gyroscopes.
2. Description of Related Art
Ring laser angular rate sensors generally comprise a ring laser within a monolithic solid block defining a ring-shaped conduit which contains an active region stimulated laser gas. Within the optical pathway of the gyroscope at least two laser beams counterpropagate in clockwise and anti-clockwise directions around the ring pathway. Over the past twenty years, the gaseous medium planar ring laser gyroscope has been developed and evolved as a reliable and relatively environmentally insensitive inertial rotation sensor. Planar ring laser gyroscopes, of both triangular and square geometries, have been used in inertial navigation systems and flight control systems regularly in both commercial and military aircraft. The primary advantage of the ring laser gyroscope over the spinning wheel mechanical gyroscope is its ability to withstand relatively large mechanical shock without permanent degradation of its performance. Because of this and other features, the expected mean time between failures of most ring laser gyroscope inertial navigation systems is several times longer than the mechanical gyroscope systems they replace.
The planar ring laser gyroscope was a first attempt at a non-mechanical truly strap-down inertial navigation system. At low rotation rates, the retroscatter from the mirrors couples energy from one of the oscillating beams into the oppositely propagating beam which locks the oscillating frequencies together yielding zero rotation information at low rotation rates. Current operational ring laser gyroscopes having a planar configuration use mechanical dithering schemes to bias the rate sensor to avoid this well known lock-in phenomenon. Mechanical dither is very effective in reducing the effects of lock-in and makes the ring laser gyroscope a viable navigational gyroscope. However, an effective mechanically dithered ring laser gyroscope adds a noise component to the output of the ring laser which in turn reduces its ultimate accuracy. Also, the presence of mechanical dither, either in the mirrors or full body dither, detracts from the desired goal of a fully strapped down inertial navigational unit.
With these problems in mind, alternative biasing techniques have been developed using the nonreciprocal Faraday effect by either applying a magnetic field to a magnetic mirror (using the Kerr effect) or directly to the gain medium (using the Zeeman effect), or to a solid glass element known as a Faraday rotator, which when used in combination with the magnetic field, provides a Faraday effect phase shift for one beam that is opposite the phase shift of the oppositely directed beam whereby two counter rotating beams are split in frequency. To achieve actual phase shifts instead of simple polarization rotation, two pairs of oppositely directed circularly polarized beams are optimally present within a single optical path to achieve a desired result. An example of this theory of multioscillator ring laser gyroscope may be found in U.S. Pat. No. 4,818,087 entitled "ORTHOHEDRAL RING LASER GYRO" issued Apr. 4, 1989 to Raytheon Corporation (Terry A. Dorschner, inventor). The nonplanar ray path produced in a multioscillator ring laser gyroscope ensures circular polarized reciprocally split light. The nonplanar ray path reciprocally rotates the polarizations by many degrees yielding the necessary circular polarization. The nonplanar reciprocal phase shift also achieves two Faraday bias gyroscopes, the gain curve G of which is illustrated in FIG. 1B. The nonplanar ray path splits the light through its geometry into two separate gyroscopes, one being left circularly polarized and the other right circularly polarized. This splitting is known as reciprocal splitting and typically is in the range of 100's of MHz. By placing a Faraday element in the beam path of a nonplanar ring laser gyroscope, when the proper magnetic field is applied to the Faraday element, nonreciprocal splitting of each gyroscope is achieved. At least four modes are produced: a left circularly polarized anti-clockwise beam (L.sub.a), a left circularly polarized clockwise beam (L.sub.c), a right circularly polarized clockwise beam (R.sub.c), and a right circularly polarized anti-clockwise beam (R.sub.a). The Faraday splitting between clockwise and anti-clockwise modes is about 1 MHz. At least four mirrors form the ring resonator path, which contains the two gyroscopes symbolized by their respective gain curves of FIG. 1B. One of the mirrors is slightly transmissive to allow light to leave the resonator and impinge upon a photo detector for signal processing. When the signals are processed electronically to remove the Faraday bias, the scale factor of the gyroscope is doubled over the conventional ring laser gyroscope. The nonplanar geometry multioscillator ring laser gyroscope using a Faraday element is currently manufactured using a gas discharge pump to provide the active medium, which occupies a portion of the light beam path.
The multioscillator ring laser gyroscope produces two signals which are optically biased (due to the Faraday cell). One signal frequency is the Faraday frequency plus one half the rate frequency; the other is the Faraday frequency minus one half the rate frequency. The gyroscope outputs the phase (integrated frequency) of these two signals. Their difference represents a rotation angle increment. However, the output signals are quantized at discrete levels separated by two .pi. of the gyroscope phase (i.e., an interference fringe).
The two output signals from the multioscillator are produced by heterodyning the like-polarized counterpropagating optical signal beams. Such signals are called heterodyne signals. One or more heterodyne signal is created for the left hand circularly polarized gyroscope and one or more is created for the right hand circularly polarized gyroscope. This can be accomplished either with an optical polarizer or other signal processing scheme. The heterodyne signals represent intensity fringes.
Optical signal fringes are produced by the heterodyne signals and detected by a pair of photo sensors. The fringes are counted and the digital counts are measures of angle increments sensed by the multioscillator ring laser sensor.
The increments of the digital angular measure of the fringes are determined by the scale factor of the ring laser sensor, and they are typically on the order of one to two arc-seconds per pulse.
The difference between the true angle and that indicated by the pulses is an error, called the quantization error or quantization noise. The ring laser sensor itself, however, is capable of measuring extremely accurate angles, and is limited by its pickoff and by the electronics processing the pickoff signal.
Usually, a resolution of one to two arc-seconds is adequate for navigation purposes; however, many new applications in the areas of pointing and tracking require even better angular resolution. These applications of the multioscillator ring laser gyroscope require very high resolution outputs, down to 0.01 or even 0.001 arc-second. There are many such applications where it is desirable to point and track with an enhanced angular resolution, such as the positioning of a terrestrial or celestial based observatory and telescope. Several techniques for refining the resolution exist, particularly by state-of-the-art digital techniques. Such techniques depend upon sampled data systems and are susceptible to aliasing errors. (The term "aliasing" is used to signify the existence of periodic function signal overlap. Aliasing is a property related to the digital sampling of continuous or discrete periodic signals. One of the main consequences of aliasing is the inability to distinguish between two periodic signals whose frequencies differ in integral multiples of the sampling rate. In this manner, when aliasing is present, the sampled data has accumulated false or exaggerated information. For digital filtering to work, aliasing must be reduced substantially or eliminated.)
Previously, attempts have been made to enhance the resolution of Ring Laser Gyroscopes (such as planar two-mode dithered model gyroscopes). In U.S. Pat. No. 4,533,250 (issued Aug. 6, 1985 to inventors, Callaghan et. al.) a READOUT APPARATUS FOR A RING ANGULAR RATE SENSOR is disclosed. The patent discloses a technique which measures the time between pulses and interpolates to determine the angle at specific times. This approach may cause problems due to excessive noise. U.S. Pat. No. 4,791,460 (issued Dec. 13, 1988 to inventors, Bergstrom, et. al. and entitled READOUT FOR A RING ANGULAR RATE SENSOR) discloses a scheme which uses a weighted sum of the analog heterodyne waveforms to generate other phase shifted waveforms thereby providing more zero crossing per cycle and hence better resolution. This approach may also encounter noise problems and is dependent of relatively precise weighing of coefficients.
Other techniques have been proposed in the past for improving the output resolution of encoders and gyroscopes. Theses include interpolation using an A/D converter to read the analog heterodyne voltages, or multipliers to create harmonics of the heterodyne signals. These techniques, however, do not have good noise immunity and are sensitive to gain, phase, and offset changes in the detectors and circuitry.
For applications where extremely fine angle resolution is required, several methods of circumventing the ring laser gyroscope readout problem were explored.
What is needed is an enhanced resolution ring laser gyroscope system so that measurements may be made down into the fractions of an arc-second with relative precision and accuracy.